Fig. 2. Different loop ties. (A) Structural. DNA repeats (green) in two cells bind to the same protein complexes (yellow ovals, red diamonds), looping the fibre. After cutting with a nuclease and removing detached fragments, the same set of repeats from each cell remain bound. When 10% DNA remains, repeats are enriched tenfold. (B) Functional. The fibre is looped by attachment to a protein complex, but both attachments and proteins in the complex change from moment to moment. After cutting and removing detached fragments, a different set of fragments remains attached in the two cells. When 10% DNA remains, no fragment is enriched tenfold. This result is obtained if cutting and removal are carried out in isotonic buffers; essentially all active transcription complexes also remain attached.